So apologies as I know the premise has been done 100 times so not much to narrow it down on but it’s a supernatural dealing with either yokai/gods(I believe specifically Norse?) girl transfers to new school or maybe mmc does? But she’s part of the underground society/government faction that takes care of supernaturals. Guy either sees her in action or almost dies/borrows her weapon which is of course a katana. I think she has an evil twin or something. I almost wanna say 9 tails is involved. I think mmc has glasses when he transforms? Early 2010s release, popular mangaka in those days

Iirc it was split into 2 parts? Whether a time skip or a second series I forget

It doesn't match all the details, but could it be Ga-rei?

Nope that’s the exact one I was looking for, thank you sir/ma’am